Aim: Equipment (Pea Pod) offering new possibilities to assess infant body composition has recently become available and has already been used in several studies. In the Pea Pod, body density is converted to body composition using one of two models ("Fomon" or "Butte") with different water content in fat-free mass (hydration factor, HF). In healthy fullterm infants we assessed HF and its biological variability in 12 newborns and calculated body composition using the two models at one and 12 weeks in 108 infants. Body weight and volume were assessed in Pea Pod and body water was assessed using isotope dilution. Results: HF was 80.9% with low biological variability (0.8% of average HF). Body fat (%) was significantly lower at one and 12 weeks when calculated using the "Butte" model than when using the "Fomon" model. The difference was more pronounced at one than at 12 weeks. Conclusion: Our HF-value agrees with that in the "Fomon" model, its low biological variability can be reconciled with the statement that Pea Pod is accurate in newborns, and "Fomon" is the best available model for studies in Pea Pod.